The group also claims to have stolen a number of "secret documents" from the church's database, which it says are now circulating on file-sharing networks.

A spokesman for the Church of Scientology, which appears to have resurrected some sites, was quoted as saying that the church was not the "right target" for such attacks.

"These types of people have got some wrong information about us," the spokesman said.

Anonymous, which has spearheaded attacks against other Web sites in the past, called on its members to rise up against the church after a video of Tom Cruise -- one of the most high-profile Scientologists -- leaked on the Internet last week.

The video, in which the Hollywood actor was seen extolling the virtues of Scientology, was reportedly taken down from some sites after the church claimed that it violated copyright.

In response, the group -- which calls itself an "Internet hate machine" -- set up a Web site to coordinate a string of attacks using phone, Internet and fax methods, which it called "Project Chanology."

The Web site asked members to bombard the church's Web site with hits so that it would collapse -- a technique known as a denial-of-service attack, as well as to exploit vulnerabilities in the site using sophisticated hacking techniques.

Anonymous also encouraged members to harass the church with phone calls, send so-called "black faxes" to use up ink and use a method called "Google bombing," where the word "Scientology" is linked to other terms such as "dangerous" and "cult" so that people entering the search term "scientology" get skewed results.

In a video posted on YouTube, which has now been watched more than 600,000 times, the group says: "With the leakage of your latest propaganda video into mainstream circulation, the extent of your malign influence over those who have come to trust you as leaders has been made clear to us.

"Anonymous has therefore decided that your organization should be destroyed, for the good of your followers, for the good of mankind, and for our own enjoyment.

"We are Anonymous. We are Legion. We do not forgive. We do not forget. Expect us."

Security experts monitoring the attacks said that the church's servers were not likely to suffer for more than a couple of days, and that the attacks were "amateur by hacking standards."

"It's a bit like the Internet equivalent of kicking in a phone [booth]," one said. "It's not that professional. In the forums where they're discussing it, some are wondering whether it will be possible to trace what they're doing."

The Church of Scientology, which is known for being litigious, has always vigorously defended itself in the wake of attacks, and points out the free choice of adults who join, as well as its efforts to help people with criminal records.